Many biological actions, for instance, response to certain stimuli and natural biological processes, are controlled by factors, such as cytokines. Many cytokines act through receptors by engaging the receptor and producing an intra-cellular response.
For example, tumor necrosis factor (TNF), both alpha and beta, are cytokines which act through TNF receptors to regulate numerous biological processes, including protection against infection and induction of shock and inflammatory disease. The TNF molecules belong to the "TNF-ligand" superfamily, and act together with their receptors or counter-ligands, the "TNF-receptor" superfamily. So far, nine members of the TNF ligand superfamily have been identified and ten members of the TNF-receptor superfamily have been characterized.
Among the ligands there are included TNF-.alpha., lymphotoxin-.alpha. (LT-.alpha., also known as TNF-.beta., LT-.beta. (found in complex heterotrimer LT-.alpha.2-.beta.), FasL, CD40, CD27, CD30, 4-1BB, OX40 and nerve growth factor (NGF). The superfamily of TNF receptors includes the p55TNF receptor, p75TNF receptor, TNF receptor-related protein, FAS antigen or APL-1, CD40, CD27, CD30, 4-1BB, OX40, low affinity p75 and NGF-receptor (Meager, A., Biologicals, 22:291-295 (1994).
All members of the TNF-ligand superfamily are expressed by activated T-cells, implying that they are necessary for T-cell interactions with other cell types which underlie cell ontogeny and functions. (Meager, A., supra).
Considerable insight into the essential functions of several members of the TNF receptor family has been gained from the identification and creation of mutants that abolish the expression of these proteins. For example, naturally occurring mutations in the FAS antigen and its ligand cause lymphoproliferative disease (WatanabeFukunaga, R., et al., Nature, 356:314 (1992), perhaps reflecting a failure of programmed cell death. Mutations of the CD40 ligand cause an X-linked immunodeficiency state characterized by high levels of immunoglobulin M and low levels of immunoglobulin G in plasma, indicating faulty T-cell-dependent B-cell activation (Allen, R. C., et al., Science, 259:990 (1993). Targeted mutations of the low affinity nerve growth factor receptor cause a disorder characterized by faulty sensory innovation of peripheral structures (Lee, K. F., et al., Cell, 69:737 (1992).
TNF and LT-.alpha. are capable of binding to two TNF receptors (the 55- and 75-kd TNF receptors). A large number of biological effects elicited by TNF and LT-.alpha., acting through their receptors, include hemorrhagic necrosis of transplanted tumors, cytotoxicity, a role in endotoxic shock, inflammation, immunoregulation, proliferation and anti-viral responses, as well as protection against the deleterious effects of ionizing radiation. TNF and LT-.alpha. are involved in the pathogenesis of a wide range of diseases, including endotoxic shock, cerebral malaria, tumors, autoimmune disease, AIDS and graft-host rejection (Beutler, B. and Von Huffel, C., Science, 264:667, 668 (1994). Mutations in the p55 Receptor cause increased susceptibility to microbial infection.
4-1BB ligand, a member of the TNF family of ligands, is induced by T-cell activation. Signalling through a 4-1BB receptor enhances proliferative T-cell responses. Among known 4-1BB receptors is the inducible murine T-cell 4-1BB receptor which is a member of the TNF receptor family. It is expressed on activated T-cells as both a 30-kDa monomer and a 55-kDa homodimer (Pollok, K. E., et al., J. Immunol., 150:771 (1993). The 4-1BB receptor binds 4-1BB ligand with a high affinity, and has been identified and cloned (Goodwin, R. G., et al., Eur. J. Immunol., 23:2631 (1993). 4-1BB ligand was highly expressed on mature B and macrophage cell lines and anti-micro-activated B-cells. Recently, the human homolog of the murine 4-1BB receptor and its ligand have been cloned (Schwarz, H. J., et al., Gene, 134:295 (1993). Data suggests a potential role for the interaction of 4-1BB receptor with its ligand in the process of T-cell activation.
A gene has also been recently identified which is induced by lymphocyte activation. The sequence of the full length 1.4 kb cDNA has been characterized as a new member of the nerve growth factor/tumor necrosis factor receptor family and is considered to be the human homolog of the murine T-cell-specific receptor 4-1BB. This receptor gene can be induced in lymphoid and differentiated non-lymphoid cell types. Expression of the protein encoded by this gene has been found on a subset of activated T or B lymphocytes. Activation-dependent expression of the protein is found not only in T lymphocytes, but also in B lymphocytes, monocytes and diverse non-lymphoid cell types (Schwarz, Blood, 85 (4):1043-1052 (1995).
The effects of TNF family ligands and TNF family receptors are varied and influence numerous functions, both normal and abnormal, in the biological processes of the mammalian system. There is a clear need, therefor, for identification and characterization of such receptors and ligands that influence biological activity, both normally and in disease states. In particular, there is a need to isolate and characterize additional NGF/TNF family receptors akin to 4-1BB which enhances proliferative T-cell responses and may be employed, therefore, for preventing, ameliorating or correcting dysfunctions or disease or augmenting positive natural actions of such receptors.